Complexity and stability in trophic networks
Soil food webs are complex networks that are typically described with plant roots, litter, and soil organic matter as the basal resources and functional groupings of microbes and invertebrates. Functional groups are groups of species that share food sources, feeding modes, life history traits and habitats. Within the networks are sub-assemblages of functional groups that form dominate pathways of material flow (aka energy channels) that originate from roots and detritus consumed by different functional groups of consumers and linked by a suite of predatory functional groups. The detritus channel can be further subdivided into a channel based on fungi and their consumers and a channel based on bacteria and their consumers. Consistent with theory, the stability of soil webs is dependent on this underlying structure of channels and on the relative dominance of activity within the channels. Less is known about how the variation within functional groups and channels affect stability, and how changes in the spatial arrangements and temporal dynamics of the basal resources affect stability. Equation-based and agent-based modeling approaches were used to study how changes in traits within functional groups interact with variations in the spatial arrangements and temporal dynamics of the inputs of basal resources to affect the dynamics and stability. The model included detritus, microbes, exo-enzymes produced by microbes, detritus-derived consumable substrates produced by exo-enzyme action, and microbial predators. For traits, dormancy and the vulnerability of dormant organisms to predation were varied. Microbes and predators possessed resting stages that they could transition to when conditions were not favorable and later emerge from. For the spatial arrangement models structured with inputs originating from roots to one structured with inputs from particles were compared. Inputs were studied comparing equal amounts entering in either a continuous or pulsed manner.
The results of the modeling exercises re-enforced the importance of the spatial arrangements and input rates of resources (bottom-up factors) and the movement patterns and consumption rates of predators on prey (top-down factors) in controlling dynamics and persistence. Apogees in the persistence of the system were observed along gradients of detritus inputs, while certain configurations were unstable under the conditions and parameterizations studied. Introducing dormancy, particularly at the base of the food web, altered the dynamics and stability of the model systems by increasing the likelihood of persistence. This is an intriguing result given the ubiquitous nature of dormancy as a trait among soil taxa.